In the preparation of steel, steel ingots are first heated to a uniform temperature and then rolled into various shapes such as billets, slabs, blooms, etc. The once rolled ingots are then reheated for further rolling. An important application for the present invention concerns the furnace in which the shaped ingots are further heated. While this type of furnace may take many different forms, it will be referred to hereinafter, generically, as a "slab reheating furnace" and the invention will be discussed with respect to a shaped ingot in the form of a slab.
The slabs move through the furnace on horizontal water cooled pipes known as skid rails which are supported from below by horizontal and vertical water cooled steel support tubes. Because it is essential that the slabs be heated uniformly, burners must be provided both above and below the level of the slabs. However, this necessitates placement of the skid rails and support tubes in the path of the hot oxidizing gases emanating from the lower burner. Thus, the immense weight of the slabs and the high temperature of the gases in the furnace place great demands upon the skid rails and the support tubes.
Clearly, the skid rails and the support tubes must be protected by sufficient insulating material to minimize the flow of heat from the interior of the furnace to the water cooled tubes. Such heat flow would reduce the operating efficiency of the furnace for at least two reasons. First it would draw off otherwise useful furnace heat, and second it would place a greater burden on the system which cools and supplies water to the water cooled tubular support members.
Conditions within the furnace place strong demands on the construction and design of the protecting and insulating material. High temperature, large temperature gradients, extreme mechanical shocks (caused by irregular or bouncing movement of the slabs along the skid rails) and the piercing effect of falling pieces of slag require that the protecting and insulating material (1) must be very thick and/or (2) must be either constituted by or at least protected by strong refractory material. It might be relatively simple to provide an adequate protecting and insulating layer on the skid rails and support tubes if no restrictions were placed upon the overall diameter of the protecting and insulating layer. However, as noted above, these tubular support members are located in the path of the hot gases between the lower burner and the slabs, as a result of which the tubular support members tend to shield or shadow the bottoms of the slabs from the heat generated by the lower burner. The effect of shadowing is to rob the lower portion of the furnace of valuable combustion space and thereby produce an uneven temperature distribution through the slab. As one manifestation of this problem, the temperature of the slabs is generally lower along the lines where the skid rails physically contact and thereby shadow the bottoms of the slabs.
As noted above, however, it is essential that the slabs be heated uniformly. Consequently, to lessen the effect of shadowing, the overall diameter of the skid rails and the support tubes, including the inner and outer protecting and insulating layers, should be kept as small as possible.
Finally, the solution to the various problems outlined above must be an economical one. To be economical, an insulating and protecting arrangement for a tubular support member must be durable (so that replacement is required only infrequently), it should maintain its structural integrity between replacements (so that furnace efficiency is not reduced for long periods between replacements) and it should be capable of easy and rapid replacement (thereby minimizing furnace shut down time).
The task of designing a truly improved skid rail and support tube protecting and insulating arrangement is complicated by the fact that the obvious solution to one problem tends to be contrary to the obvious solution to another problem. For example, good heat retention in the furnace requires a thick insulation but this only increases the detrimental effect of shadowing. The prior art to date is composed mainly of compromise solutions which have solved some of the problems, either to the detriment of, or while ignoring the other problems.
An early attempt to provide a suitable heat barrier between the furnace and the water cooled tubes included surrounding the tube with a layer formed by a plurality of large, rigid sections formed of a refractory material, leaving a space between the tube and the inner surface of the refractory layer. An example of this arrangement is shown in the Schmidt U.S. Pat. No. 2,436,452. However, this arrangement requires that the large sections (1) be mounted on studs or other mounting members in direct heat transfer relationship with the tube and (2) be rigidly held to each other. Because of the tight rigid fit between the sections, the refractory layer cannot yield so as to withstand the intense vibrational shocks caused by movement of the slabs; and because of the discrepancy in heat flow characteristics between the studs and the refractory material, large thermal gradients are established and these in turn cause thermal stresses. The effect of the vibrational shocks and the thermal stresses is to cause cracking of the large section. On the other hand, it is not always feasible to eliminate the tight fit between the sections since this would create air spaces between the sections for the flow of heat and gases from the furnace to the water cooled tube. Moreover, because the gases in the furnace are oxidizing in nature, they tend to corrode the tube. Another disadvantage of the large sections is that they are normally held together in a complex arrangement, thereby rendering replacement of a single section quite difficult without replacing the entire layer.
In another prior art arrangement, the large sections are replaced by a one-piece refractory layer molded directly onto the tube and held in place by suitable means, such as a wire mesh welded to the tube. An example of this arrangement is shown in the U.S. Pat. No. 2,693,352 to Bloom. This arrangement does not eliminate the cracks derived from thermal stresses and vibrational shocks. It does provide the advantage that at least the pieces are held in place on the tube for a longer period of time by the wire mesh. However, the molding approach creates a problem since the replacement time for molding a layer of refractory material would be relatively long, thereby increasing down time of the furnace.
Still another prior art approach includes arranging a plurality of sectional refractory elements partially circumferentially about the water cooled tube, holding the sections in place with mounting elements connected to the tube and with tongue and groove connections between the sections, and placing a layer of ceramic fiber insulating material in the annular space between the refractory layer and the tube. A circumferential gap is left between the last two elements and this gap is closed with a refractory mortar. Such an arrangement is shown, for example, in U.S. Pat. No. 3,226,101 to Balaz. However, this arrangement would apparently suffer from the same disadvantages as the earlier approaches which employed direct heat flow passages in the form of mounting means between the furnace and the water cooled tube. Moreover, if the refractory elements are held tightly together, it would appear that this arrangement would not accommodate the vibrational shocks caused by movement of the slab along the rails. On the other hand, as noted earlier, if the refractory elements are not held tightly together, additional passages would be provided between the sections for heat loss and for the flow of corrosive gases from the furnace to the metal tube. The mortar closure usually lacks durability, and the service life of this arrangement often leaves much to be desired.
Another modification of insulation is shown in U.S. Pat. No. 3,451,661 to Barker. This describes an inner layer of resilient insulating material which is held in place by an outer layer of interlocking refractory elements. While a resilient inner body of insulation is provided by this method, the outer layer elements are of rigid material and subject to breakage, while their assembly is somewhat complicated and time consuming. In this type of furnace the insulation must withstand the combination of high thermal and mechanical stresses and yet should permit rapid installation and repair; the insulation arrangements proposed so far have not been entirely satisfactory in these regards.